It is known that it is possible, with or without inserting free carriers, to modify locally the forbidden band energies and the refractive indices of such heterostructures by techniques of interdiffusion or of alloy disordering. Such modifications of forbidden band energies and of refractive indices give rise to modifications to the electro-optical properties of the heterostructures, and make it possible to provide various electro-optical and/or photonic functions that may be active (laser, modulator, etc.) or passive (waveguide, directional coupler, etc.).
Of the techniques most widely in use at present, mention may be made of the techniques of thermal diffusion or of ion implantation, and also of techniques of depositing dielectric layers.
In the thermal diffusion technique, disorder is created or the opto-electronic properties of the heterostructure are transformed by inserting an n-type electrically active element such as silicon (Si) or sulfur (S), or by inserting a p-type element such as zinc (Zn) for example. Insertion is obtained after heat treatment that induces diffusion of the element which is either deposited directly on the surface, or which makes contact with the surface as a vapor at the heat treatment temperature. For examples of applications of that technique, reference may advantageously be made to the following various publications:
Low-threshold InGaAs/GaAs/AlGaAs quantum well laser with an intracavity optical modulator by impurity-induced disordering; W. X. Zou et al., Applied Physics Letters, 62 (6), 1993, pp. 556-558;
Impurity-induced layer disordering in In(AlGa)P-InGaP quantum well heterostructures: visible spectrum buried heterostructure lasers; J. M. Dallesasse et al., Journal of Applied Physics Letters, 66 (2), 1989, pp. 482-487;
GaAlAs buried multiquantum well lasers fabricated by diffusion-induced disordering; Tadashi Fukuzawa, Shigeru Semura, Hiroshi Saito, Tsuneaki Ohta, Yoko Uchida, and Hisao Nakashima; Appl. Phys. Lett. 45 (1), Jul. 1, 1984, pp. 1-3; and
Fabrication of GaAlAs "window-strip" multi-quantum-well heterostructure lasers using Zn diffusion-induced alloying; Y. Suzuki, Y. Horikoshi, M. Kobayashi, H..sub.-- Okamoto, Electronics Letters, Apr. 26, 1984, Vol. 20, No. 9, pp. 383-384.
That technique nevertheless suffers from the drawback of being usable only when it is desired to obtain transformed regions that are electrically doped, i.e. with the presence of n-type or p-type free carriers.
In particular, it will be observed that in the above-mentioned article by J. M. Dallesasse et al., the layer which is deposited on the quantum well structure is a layer of Si and not a layer of doped SiO.sub.2. The layer of doped SiO.sub.2 that is mentioned is a layer that encapsulates the sample constituted by the quantum well layer and the Si layer.
The layer of doped SiO.sub.2 therefore does not intervene directly in the modification of the energy levels of the quantum well heterostructure; it is the diffusion of Si in the heterostructure that gives rise to disordering and to the transformation of the opto-electronic properties thereof.
The diffusion of silicon inserts free carriers into the disordered or modified region.
In the ion implantation technique, a mask is placed on the surface of the heterostructure to define regions that are to be modified locally, and bombardment is then performed using the ions that are to be implanted. The heterostructure is then subjected to heat treatment.
In this respect, reference may advantageously be made to the following two publications:
Quantum well laser with integrated passive waveguide fabrication by neutral impurity disordering; S. R. Andrew et al., IEEE Photonics Technology Letters, Vol. 4, No. 5, 1992, pp. 426-428; and
Large blueshifting of InGaAs/InP quantum well band gaps by ion implantation; J. E. Zucker et al., Applied Physics Letters, 60 (4), 1992, pp. 3036-3038.
Although ion implantation makes it possible to transform the heterostructure with or without inserting free carriers into the transformed region, that technique causes residual defects to appear in the transformed regions of the heterostructure, which defects are difficult to eliminate. As a result, ion implantation techniques have so far been used only for implementing passive functions.
Dielectric layer techniques consist in depositing dielectric layers such as SiO.sub.x or Si.sub.x .multidot.N.sub.y .multidot. on quantum well heterostructures and then in heat treating the resulting samples to obtain transformation without inserting free carriers. In the case of quantum well heterostructures based on GaAs, for example, the heat treatment causes the gallium to be exodiffused into the SiO.sub.x dielectric, thereby generating Ga gaps in the dielectric-structure interface. Deep thermal diffusion of those gaps gives rise to the transformation of the quantum heterostructure.
For examples of applications of those techniques, reference may advantageously be made to the following various publications:
Monolithic waveguide coupled cavity lasers and modulators fabricated by impurity-induced disordering; Robert L. Thornton, et al., Journal of Lightwave Technology, Vol. 6, No. 6, 1988, pp. 786-792;
Integrated external cavity InGaAs/InP lasers using cap-annealing disordering; T. Miyazaza et al., IEEE Photonics Technology Letters, Vol. 3, No. 5, 1991, pp. 421-423;
Refractive index change of GaInAs/InP disordered superlattice waveguide; A. Wakatsuki et al., IEEE Photonics Technology Letters, Vol. 3, No. 10, 1991, pp. 905-907;
Monolithic integration of an (Al)GaAs laser and an intracavity electroabsorption modulator using selective partial interdiffusion; S. O'Brien et al., Applied Physics Letters, 58 (13), 1991, pp. 1363-1365; and
Spatial control of quantum well intermixing in GaAs/GaAlAs using a one-step process; S. G. Ayling et al., Electronics Letters, Vol. 28, No. 24, 1992, pp. 2240-2241.
Because of the stresses at the interface between the dielectric and the quantum well heterostructure sample, that dielectric deposition technique is difficult to apply to reproducible fabrication of discrete or integrated components based on GaAs. With InP, it is not applicable in reproducible manner. Only one implementation is reported in the literature and it was obtained under particularly severe conditions of heat treatment by successive pulses of fast temperature rises.